In areas where municipal treatment of wastewater is unavailable, septic tank systems are usually relied upon. In a typical septic system, raw sewage containing a significant concentration of waste solids, enters one end of an in-ground tank and discharges at a discharge end of the tank. In the tank, solids separate from the liquid in the waste water. Solids in the form of scum collect on the surface while solids more dense than the liquid sink to the bottom as a sludge. The liquid between the surface and the sludge which is relatively clear, exits the discharge end of the tank.
The condition of the effluent from the septic tank determines the appropriate secondary treatment. This condition is measured by the biochemical oxygen demand (BOD), total suspended solids (TSS) and total nitrogen present in the effluent. The BOD and local nitrogen concentrations of the septic tank effluent primarily determine the size and kind of the secondary waste treatment facilities. Decomposition of organic material by bacteria present in the anaerobic environment of a septic tank changes the organic nitrogen to ammonia nitrogen. Thus, the septic tank effluent contains nitrogen primarily as ammonia nitrate. An aerobic treatment which typically occurs in the secondary treatment reduces BOD and TSS to acceptable levels prior to discharge of the secondary effluent. In this treatment bacteria oxidize ammonia nitrogen to nitrate nitrogen. Thus, in conventionally treated septic tank effluent nitrogen is present primarily as ammonia nitrogen. In an aerobic environment, such as in secondary treatment, the bacteria oxidize ammonia nitrogen to nitrate nitrogen.
Vandervelde et al. in U.S. Pat. Nos. 4,997,568 and 5,632,896 discloses a system of treating septic tank effluent which consists of infusing the effluent at extremely low velocities through a plurality of precisely graded progressively finer pored media, starting with a larger interface area and reaction volume and then with smaller average interstitial distance, and increasingly more intimate contact between biota and pollutants. A progressive treatment is achieved by shaping the reactor in a cylinder, wedge or cone shape with the largest pored media at the center or apex where the effluent is introduced. The effluent then seeps radially to the periphery while being purified. Contact with ambient air can be maximized by shaping the filter bed to have a large air-effluent interface.
One can use coarse gravel in the lower filter beds or even open cell foam plastic or other porous materials. It is also known to use short sections of pipe randomly oriented in place of the coarse gravel. The purpose of the parts in the filtering media is to provide a surface to which microorganisms can attach and function to react with the effluent. Fabco Plastics of Maple, Ontario manufactures and sells short sections of plastic saddles with many small openings forming a porous complex part and serrated edges and parts having a short cylindrical structure with large openings and radially spaced, diametrically extending supports. It is not clear that such designs offer a significantly increased surface area for microorganisms to attach to as each opening reduces the surface area by the size of the opening but adds the surface area of the edges around the opening. It is expected that such parts are costly to manufacture by injection molding given their complexity.
Accordingly, it is an object of the invention to provide an improved part for a filtering media. It is a further object to provide a part for a filtering media that has a relatively simple construction so that it is easily injection molded while, at the same time, being inexpensive to produce.